High density element structure formed by assembly of layers and method for making same

ABSTRACT

The invention relates to a structure comprising a sequence of elements (t) for sending or receiving a signal along an axis. Two successive elements (t) along the direction of the axis are offset with respect to each other along the direction perpendicular to the axis. The structure comprises at least two layers of material (K 1,  K 2 ) deposited on a reception substrate (S 1 ) using the layer transfer technique.  
     The invention particularly relates to any type of structure for which elements must have a high density (print head, networks of optical components, antenna networks, etc.).

TECHNICAL FIELD AND PRIOR ART

[0001] This invention relates to a structure comprising a sequence ofelements for sending or receiving a signal along an axis.

[0002] The invention may advantageously be used in the case in which theelements that will send or receive the signal must have a high density.For example, it may be applicable to networks of optical components(laser diodes, optical fibres, detectors), antenna networks, printheads, etc.

[0003] By way of a non-restrictive example, the invention will bedescribed specifically for a print head application.

[0004] One known print technique is printing using arrays. Each arraycomprises elements aligned with each other side by side. Each element iseither a magnetic head or a resistance, depending on whether the printsignal is magnetostatic or thermal. One or several arrays placed end toend form a line with the same width as the support to be printed.

[0005] The support on which the printing will be done moves with respectto the arrays that transform the received electrical write signals intoeither magnetic signals or thermal signals. The support is printed lineby line by relative movement of the support or the arrays. Each elementreceives a write signal that is repeated for each line to be printed.

[0006] Several processes for manufacturing print heads are knownaccording to prior art.

[0007] A first known technique consists of assembling individual printheads. For example, an individual print head may comprise a magnetichead controlled by a diode or a transistor. The magnetic head is made ona mechanical support and the control diode is welded on the support.Individual print heads are then mounted on a mechanical support with aninsertion part between heads.

[0008] This manufacturing method limits the resolution of the print headto values between 150 and 300 dpi (dots per inch).

[0009] A second manufacturing method is related to microelectronicstechniques. The print heads are then made collectively on asemiconducting substrate. One example of the collective structure ofprint heads obtained according to this second method is shown in FIG. 1.

[0010] The print head 1 is composed of a set of individual magneticheads 3 made on a semiconducting substrate 4. Each individual magnetichead 3 is controlled by a diode 2. The diodes 2 may be integrated oradded onto the semiconducting substrate 4.

[0011] Resolutions of the order of 600 dpi can be achieved using thissecond manufacturing technique. However, such resolutions cannot bereached unless the size of the magnetic heads is reduced. Unfortunately,this results in a reduction in the intensity of the magnetic fieldinduced by the magnetic heads.

[0012] This technique also has other disadvantages.

[0013] As shown in FIG. 2, the drum 5 of the printer comes into contactwith magnetic heads 3. If the diodes 2 are added onto the substrate 4,then the diodes 2 need to be moved away from the heads 3 to prevent thedrum from coming into contact with the diodes. The zone between thediodes 2 and the heads 3 is then lost. The electrical resistance of thelength of the line separating one diode 2 from a magnetic head 3 canthen reduce the performances of the magnetic head.

[0014] If the diodes 2 are integrated into substrate 4, the disadvantagementioned above no longer exists. However, in this case two differentmanufacturing technologies have to be implemented; one for the diodesand another for the magnetic heads. Consequently, the manufacturingefficiency is reduced.

[0015] The invention does not have the disadvantages mentioned above.

[0016] Presentation of the Invention

[0017] The invention relates to a structure comprising a sequence ofelements to send or receive a signal along an axis. At least twosuccessive elements along the direction of the axis are offset from eachother, in a direction perpendicular to the axis.

[0018] According to the preferred embodiment of the invention, thestructure comprises at least one set of at least two layers of materialdeposited on a reception substrate, each layer of material comprising asequence of elements in line with each other along a direction parallelto the axis, two successive elements of the structure along thedirection parallel to the axis belonging to different layers.

[0019] The invention also relates to a print head comprising a sequenceof magnetic heads or resistances to print along an axis. The print headis a structure like that according to the invention.

[0020] The invention also relates to a process for manufacturing astructure comprising a sequence of elements to send or receive a signalalong an axis. The process comprises an assembly step of at least twomaterial layers each comprising a sequence of elements aligned along adirection parallel to the axis, the assembly being made such that twosuccessive elements of the structure along the direction parallel to theaxis belong to two different layers.

[0021] According to the preferred embodiment of the invention, thematerial layers are assembled by transferring layers from a transfersupport to a reception substrate and by putting the transferred layersinto bonding contact.

[0022] The invention also relates to a process for manufacturing a printhead comprising a series of magnetic heads or resistances to make aprintout along an axis. The process implements a process according tothe invention as described above.

[0023] The invention also relates to a process for manufacturing astructure comprising a sequence of elements to send or receive a signalalong an axis, two successive elements along the direction of the axisbeing offset, one with respect to the other, along a directionperpendicular to the axis, characterised in that it comprises thefollowing steps:

[0024] deposition of at least one semiconducting layer on a firstsemiconducting substrate, comprising a set of elements aligned with eachother to send or receive a signal along the axis,

[0025] transfer from a transfer support onto a free face of asemiconducting layer deposited on the first substrate,

[0026] withdrawal of the first substrate,

[0027] transfer using the transfer support, from the free face of thesemiconducting layer that appeared after the first substrate waswithdrawn on a first free face of a semiconducting layer fixed on areception substrate and comprising a set of elements aligned with eachother to send or receive a signal along the axis,

[0028] molecular bonding on the two faces brought into contact bytransferring, using the transfer support.

[0029] Advantageously, the invention provides a high resolutionstructure. For example, it is possible to achieve resolutions of theorder of 1200 dpi or more, by successive layer transfers.

[0030] The process according to the preferred embodiment of theinvention uses the layer (or multiple layer) transfer technique bymolecular bonding on wafer scale. This technique consists of making atleast one semiconducting layer on a first substrate comprising printheads, and then transferring the semiconducting layer(s) comprising theprint heads by molecular bonding, onto a second substrate on which thereis already at least one layer comprising print heads; thus, the numberof levels of print heads of the structure can then be increasedsuccessively. Molecular bonding enables the manufacture ofinterconnections using microelectronics techniques (plasma etching,sputtering). Depending on the structures made, it is possible to bringall input/outputs onto a single face or onto two faces. Furthermore, thelayer transfer technique is compatible with the mixing process formeltable beads.

[0031] This technique is also compatible with wafer thinning methods,which can further reduce the dimensions of the print module.

[0032] The distance between two consecutive heads may be reduced to thethickness of the layers, so that a vertical pitch equivalent to thehorizontal pitch can be obtained.

[0033] This method also enables the assembly of several functions(layers for heads and layers for control circuits) and interconnectionsbetween the heads and control circuits (still using microelectronicsprocesses).

[0034] When transferring onto a back face, the advantage of transfer bymolecular bonding means that print heads can be made on both sides suchthat the layers on each face do not come into contact with the equipmentsubstrate holders.

[0035] Compared with assembly techniques according to prior art, theassembly technique by transfer of layers according to the preferredembodiment of the invention advantageously initially makes it possibleto make a stack of heads on a first support so that the precision ofmicroelectronics technologies can be used, and then to transfer thestack of heads onto another support adapted to the required application.For example, the first support may be a wafer made of silicon, AsGa,SiC, SOI or InP. For example, the support adapted to the requiredapplication may be a plastic substrate for a print head application witha drum system (which then avoids deterioration of the substrate due tofriction of the substrate on the drum), a ceramic substrate for a headusing a thermal effect, a printed circuit to connect a stack of heads toother components such as control chips, or a flexible printed circuit.

[0036] The assembly technique according to the invention thus makes itpossible to integrate all functions (print heads, control circuits andinterconnections) on one or two supports depending on the variant, whileminimizing the dimensions of the module (global dimensions and distancebetween heads).

[0037] In the remainder of the description, the invention will bedescribed specifically for the case in which the structure is a printhead and the elements are magnetic heads. More generally, as alreadymentioned above, the invention relates to any structure comprising asequence of elements to send or receive a signal along an axis: networkof optical components (laser diodes, optical fibres, detectors), antennanetworks, etc.

BRIEF DESCRIPTION OF THE FIGURES

[0038] Other characteristics and advantages of the invention will becomeclear after reading embodiments of the invention made with reference tothe appended figures among which:

[0039]FIG. 1 shows a print head according to prior art,

[0040]FIG. 2 shows a sectional view of the print head in FIG. 1,equipped with its drum,

[0041]FIG. 3 shows a perspective view of a first embodiment of a printhead according to the invention,

[0042]FIG. 4A shows a longitudinal sectional view of a print head andFIG. 4B shows a transverse view of a substrate comprising print headsaccording to the invention,

[0043]FIGS. 5A to 5F show a process for manufacturing a print headaccording to the second embodiment shown in FIGS. 4A and 4B,

[0044]FIGS. 6A to 6F show different examples of print heads according tothe invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0045] The same marks denote the same elements in all figures.

[0046]FIGS. 1 and 2 have already been described, and therefore there isno point in describing them again.

[0047]FIG. 3 shows a perspective view of two subassemblies used to makea first example of a print head according to the invention.

[0048] A first subassembly E1 comprises a first array 6, a second array7 and several blocks of diodes B1, B2 and B3. A second subassembly E2,identical to the first subassembly E1, comprises a first array 8, asecond array 9 and several blocks of diodes (not shown in the figure).

[0049] Arrays 6 and 7 of subassembly E1, and arrays 8 and 9 ofsubassembly E2, are fixed to each other, for example by solder beadsdeposited on anchor studs. These solder beads also make an electricalconnection between the arrays 6 and 7 of subassembly E1 and arrays 8 and9 of subassembly E2.

[0050] Each array comprises a set of magnetic heads made on one of itsfaces. The face on which the magnetic heads of array 6 are located isfixed to the face on which the magnetic heads of array 7 are located.Similarly, the face on which the magnetic heads of array 8 are locatedis fixed to the face on which the magnetic heads of array 9 are located.

[0051] In order to fix the array 7 (or 9) to array 6 (or 8), the array 7(or 9) is located on the array 6 (or 8), and the parts align themselveswith each other during the solder bead remelting phase.

[0052] The control diodes of the magnetic heads in the first subassemblyE1 are made in blocks (B1, B2, B3) added onto the face of the array 6 onwhich the magnetic heads are made. The diodes of blocks B1, B2, B3 areused to control the magnetic heads of array 6 and the magnetic heads ofarray 7 at the same time.

[0053] Similarly, the control diodes for the magnetic heads in thesecond subassembly E2 are made in blocks (not shown in the figure) addedonto the face of the array 8 on which the magnetic heads are made.

[0054] The diodes in the blocks thus added on are used to control themagnetic heads of arrays 8 and 9.

[0055] The two subassemblies E1 and E2 are assembled to each other usingspacers T, for example rigid beads, placed in cavities C previouslyformed on the back faces of the corresponding arrays 6 and 8.

[0056] The two subassemblies E1 and E2 are pressed into contact witheach other and aligned. The remaining space between the modules can thenbe filled in with glue, for example epoxy resin.

[0057] The subassemblies E1 and E2 may also be assembled by meltablebeads added onto anchor studs. These anchor studs are then firstly madeon the back faces of the corresponding arrays 6 and 8.

[0058] The remelting temperature of the beads used to assemblesubassemblies E1 and E2 is less than the remelting temperature of thebeads used for assembly of arrays 6 and 7 (or 8 and 9). The twosubassemblies become aligned with each other during the melting phase.

[0059] Regardless of the stack made, the assembly at the end of theprocess may advantageously be filled, for example, with resin to give amechanically rigid assembly (similarly for other example embodiments).

[0060] The ends of the magnetic heads of subassemblies E1 and E2 formmagnetic poles. By way of a non-restrictive example, the subassembly E1comprises 14 magnetic poles PE1 ₁, . . . , PE1 ₁₄ and subassembly E2comprises 14 magnetic poles PE2 ₁, . . . , PE2 ₁₄. According to theinvention, the signals that form a print line on the support are thencomposed, for example, of signals output from a sequence of magneticpoles PE1 ₁, PE2 ₁, PE1 ₂, PE2 ₂, . . . , PE1 ₁₄,PE2 ₁₄.

[0061] The difference in height between poles in the directionperpendicular to the axis that defines the print line may be compensatedby an electronic time shift system.

[0062] By way of a non-restrictive example, for a pitch of magneticpoles that gives a resolution of 225 dpi for an array (6, 7, 8 or 9), aresolution of 450 dpi can be obtained for each subassembly E1, E2 byassembling the corresponding arrays of these subassemblies with anoffset of a half-pitch. An assembly of two sub-assemblies E1 and E2 witha quarter-pitch offset then gives a resolution 900 dpi. This resolutioncan then be further increased if the pitch of the magnetic poles in anarray gives a resolution better than 225 dpi. For example, a resolutionof 300 dpi for arrays 6, 7, 8 and 9 gives a resolution of 1200 dpi(4×300 dpi) for the print head composed of the two subassemblies E1 andE2.

[0063] An offset in the magnetic poles along a direction parallel to theprint axis may be made in several ways. For example it is possibleeither to make different arrays with different positions of the magneticpoles and then assemble the arrays symmetrically, or to make identicalarrays and then assemble the arrays offset from each other.

[0064]FIGS. 4A and 4B show a longitudinal sectional view of a print headand a transverse sectional view of a substrate comprising print headsaccording to the invention, respectively.

[0065] Substrate S1 is covered by a layer K1 that is itself covered by alayer K2. Each layer Ki (i=1, 2) is composed of a material 10 in whichthe magnetic heads t are formed. An increase in the resolution isobtained by an offset of the magnetic heads t (see FIG. 4B).Advantageously, the distance h that separates two successive magneticheads along the direction perpendicular to the print axis may be as highas of the order of 20 μm to 30 μm. This value of h may be approximatelyequal to the minimum distance d between two successive magnetic heads inthe print head. In this case, no electronic compensation of signalsapplied to magnetic heads is necessary.

[0066] The print head described in FIGS. 4A and 4B only contains twolayers K1 and K2 equipped with magnetic heads. However, note that moregenerally, the invention relates to a print head comprising at least twolayers equipped with magnetic heads.

[0067]FIGS. 5A to 5F show an example of a process for manufacturing aprint head according to the invention.

[0068]FIG. 5A shows a structure composed of an initial substrate I onwhich a stop layer 11 and a layer K2 equipped with magnetic heads t, aredeposited. The substrate I enables the formation of layers 11 and K2according to microelectronics technologies. The substrate I may forexample be a wafer of silicon, AsGa, SiC, SOI or InP. The stop layer 11may for example be a silicon oxide.

[0069] By way of a non-restrictive example, the structure shown in FIG.5A comprises a stop layer 11 and a layer K2 equipped with magnetic headst. The invention also relates to the case in which there is no stoplayer 11 and/or the case in which the structure comprises a stack ofseveral layers equipped with magnetic heads.

[0070] The process for manufacturing of a print head starting from astructure such as that shown in FIG. 5A comprises the following steps:

[0071] transfer of a transfer support t onto the free face of the layerK2, for example by bonding. The transfer support T, commonly called ahandle, may be made from a transparent material, for example glass orpure silica, to avoid alignment problems (FIG. 5B),

[0072] withdrawal of the initial substrate I by a conventional processor a combination of conventional processes such as mechanical grinding,polishing, separation along a cleavage plan induced by ionicimplantation, chemical etching, reactive, selective etching orultrasound etching (FIG. 5C),

[0073] removal of the stop layer 11 (FIG. 5D),

[0074] transfer of the free face of layer K2 onto the free face of alayer K1, the other face of which is fixed to a substrate S1, and thetwo faces being brought into bonding contact for example by gluing ormolecular bonding; the gluing energy may be increased by carrying outsurface treatments on the surfaces to be glued,

[0075] removal of the transfer support T by one of the processesmentioned above in the description of FIG. 5C (FIG. 5F).

[0076] The process according to the invention also comprises steps inwhich electrical contact is made on the magnetic heads (opening in thepassivation and metallisation layers) These steps are not shown in thefigures.

[0077] By way of a non-restrictive example, layers K1 and K2 may be madestarting from the semiconducting material. The layer K1 can then betransferred onto layer K2 using the molecular bonding technique.Molecular bonding concerns two bonding types; hydrophilic bonding andhydrophobic bonding. Hydrophilic bonding is the result of a change to—OH interactions at the surface of the structure, for example to formSi—O—Si bonds. The forces associated with this type of interaction arestrong. The bonding energy, of the order of 100 mJ/m² at ambienttemperature, can be as high as 500 mJ/m² after annealing at 400° C. for30 minutes (values obtained by native or hydrophilic SiO2—thermalunpolished SiO2 bonding).

[0078] The bonding energy is usually determined using the strip methoddivulged by W. P. MASZARA et al. in the “Bonding of silicon wafers forsilicon-on-insulator” article published in J. Appl. Phy. 64(10), Nov.15, 1988, pages 4943-4950. The bonding energy for bonding between adeposited and polished silicon oxide and a deposited and polishedsilicon oxide is of the order of 1 J/m² for annealing under the sameconditions. However, if a hydrophilic treated surface is bonded onto ahydrophobic treated surface by molecular bonding, the bonding qualityobtained is very poor and the bonding forces are very low:bonding energyof the order of 100 mJ/m² after annealing at 400° C. for 30 minutes.

[0079] With this bonding method, two substrates comprisingmicroelectronics technologies can be bonded provided that their surfaceconditions are prepared. The precision on the pitch of two successivemagnetic heads is advantageously obtained by the bonding precision,namely ±approximately 1 μm both along the direction of the print axisand along the direction perpendicular to the print axis.

[0080]FIGS. 6A to 6F show different examples of print heads according tothe invention.

[0081]FIGS. 6A and 6B show a longitudinal view and a transverse viewrespectively of a structure composed of a substrate S2 on which fourlayers K3, K4, K5 and K6 are stacked in sequence.

[0082]FIGS. 6C and 6D show two examples of print heads comprisingstructures assembled using connections by solder beads. Those skilled inthe art know how to make connections by solder beads referred to as the“flip-chip” technique.

[0083]FIG. 6C shows an assembly of two structures. The first structureis composed of a substrate S3 on which two layers K7 and K8 are bondedin sequence. A second structure is composed of a substrate S4 on whichtwo successive layers K9 and K10 are bonded. The connection by solderbeads is made between the free faces of layers K8 and K10. For example,this structure can give a resolution of the order of 1200 dpi.

[0084] The print head shown in FIG. 6D comprises the association ofthree structures. A first structure is composed of a substrate S5covered on a first face by two layers K13 and K14, and on the other faceby two layers K15 and K16. The other two structures are each composed ofa substrate (S3, S6) on which two layers are bonded (K7 and K8 forsubstrate K3; K15 and K16 for substrate S6). The free faces of layersK16 and K14, and the free faces of layers K8 and K12, are assembled bysolder beads.

[0085]FIGS. 6C and 6F show two examples of print heads according to theinvention including diodes for control of the magnetic heads.

[0086]FIG. 6E shows a structure with two layers K17 and K18, each layercomprising a set of diodes D for controlling the magnetic headscontained in it. FIG. 6F shows a structure with two layers K19, K20. Adiodes block BD is connected by solder beads b to the free face of theupper layer K20.

[0087] FIGS. 6A-6F are given by way of non-restrictive examples. Moregenerally, as already mentioned above, the invention relates to anystructure comprising at least one assembly composed of a receptionsubstrate on which at least two layers of material, each provided withmagnetic heads, are deposited. The assemblies composed of a receptionsubstrate and layers of material can then be connected to each other bysolder beads or by any other means.

1. Structure comprising a sequence of elements (t) to send or receive asignal along an axis, two successive elements (t) along the direction ofthe axis being offset from each other along a direction perpendicular tothe axis, characterised in that the structure comprises at least one setof at least two layers of semiconducting material (K1, K2, K3, K4)deposited on a reception substrate (S3), each layer of semiconductingmaterial comprising a sequence of elements in line with each other alonga direction parallel to the axis, two successive elements of thestructure along the direction parallel to the axis belonging to twodifferent layers, the layers of semiconducting material in at least onset of layers being brought into bonding contact by molecular bonding.2. Structure according to claim 1, characterised in that a firstassembly is connected to a second assembly by solder beads (b). 3.Structure according to claim 1 or 2, characterised in that at least onereception substrate (S5) is covered with at least two layers of material(K11, K12) on a first face and with at least two layers of material(K13, K14) on a second face.
 4. Structure according to any one of claims1 to 3, characterised in that it comprises at least one set of diodes inthe form of a chip (BD) connected by solder beads onto a face of a layerof material.
 5. Structure according to any one of claims 1 to 4,characterised in that it comprises at least one set of diodes (D) in theform of diodes integrated into a layer of semiconducting material. 6.Print head comprising a sequence of magnetic heads or resistances (t) toprint along an axis, characterised in that it is a structure accordingto any one of claims 1 to 5, in which an element of the sequence ofelements is a magnetic head or a resistance.
 7. Process formanufacturing a structure comprising a sequence of elements (t) to sendor receive a signal along an axis, the process comprising a step for theassembly of at least two layers of semiconducting material (K1, K2),each comprising a sequence of elements aligned along a directionparallel to the axis, the assembly being made such that two successiveelements of the structure along the direction parallel to the axisbelong to two different layers, the assembly step comprising thetransfer of a second layer of semiconducting material (K2) onto a firstlayer of semiconducting material (K1) in order to bring a first face ofthe first layer (K1) into bonding contact with a first face of thesecond layer (K2), by molecular bonding.
 8. Process according to claim7, characterised in that the second layer of material (K2) istransferred onto the first layer of material (K1) by means of a transfersupport (T) fixed to the second layer (K2).
 9. Process according toclaim 8, characterised in that it comprises removal of the transfersupport (T), once the layers have been put into bonding contact. 10.Process according to any one of claims 7 to 9, characterised in that itcomprises a connection step using solder beads between a first assemblycomposed of a first reception substrate (S5) onto which at least twolayers of material (K11, K12) are fixed by bringing them into bondingcontact, and at least one second assembly composed of a second receptionsubstrate (S3, S6), on which at least two layers of material (K7, K8;K15, K16) are fixed by bringing them into bonding contact.
 11. Processaccording to any one of claims 7 to 10, characterised in that itcomprises the connection of at least a set of diodes (BD) onto at leastone layer of material by solder beads, each diode in the set of diodesenabling control of an element (t) of the layer.
 12. Process accordingto any one of claims 7 to 11, characterised in that it comprises theformation of at least a set of diodes (D) in at least one layer ofsemiconducting material, each diode in the set of diodes enablingcontrol of one element (t) in the layer of semiconducting material. 13.Process for manufacturing a print head comprising a sequence of magneticheads or resistances (t) to make a printout along an axis, characterisedin that it uses a process according to any one of claims 7 to 12, inwhich one element of the sequence of elements is a magnetic head or aresistance.